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Earthquake Damage in Mexico City, Mexico, September 19, 1985

On September 19, 1985, a magnitude 8.1 earthquake occurred off the Pacific coast of
Mexico. The damage was concentrated in a 25 km2 area of Mexico City, 350 km from the
epicenter. The underlying geology and geologic history of Mexico City contributed
to this unusual concentration of damage at a distance from the epicenter. Of a population
of 18 million, an estimated 10,000 people were killed, and 50,000 were injured. In
addition, 250,000 people lost their homes and property damage amounted to $5 billion.
This set of slides shows different types of damaged buildings and the major kinds
of structural failure that occurred in this earthquake including collapse of top,
middle and bottom floors and total building failure. The effect of the subsoils on
the earthshaking and building damage are emphasized. Over 800 buildings crumbled,
including hotels, hospitals, schools, and businesses. Communications between Mexico
City and the outside world were interrupted for many days. Surrounding areas affected
by the earthquake included the Mexican States of Jalisco, Guerrero, and Michoacan.
Damage in the epicentral area wasrestricted to a few tourist resorts and industrial
estates along the Mexico Pacific coast. A two-meter tsunami also caused some damage
in this area. There are geologic reasons why Mexico and especially Mexico City are
vulnerable to earthquake damage. Along the west coast of southern Mexico and Central
America, the Cocos Plate dips beneath the North American Plate producing a very active
seismic zone. Since the beginning of the Twentieth Century, 84 earthquakes of magnitude
greater than 7.0 have occurred in this zone. The location of the 1985 earthquake's
epicenter near the coast at the border between the states of Michoacan and Guerrero
was not a surprise. Prior to the 1985 earthquake this area, located between two areas
that had experienced recent earthquakes, was known as the "Michoacan Gap." The "gap"
was filled in 1985 by the main shock and a severe aftershock (magnitude 7.5) that
occurred two days later, on September 21. Mexico City lies in a broad basin formed
approximately 30 million years ago by faulting of an uplifted plateau. Volcanic activity
closed the basin and resulted in the formation of Lake Texcoco. The Aztecs chose an
island in this lake as an easily defended location for their capitol. The expansion
of Mexico City and the gradual draining of the lake left the world's largest population
center located on unconsolidated lake-bed sediments. These soft sedimentary clay deposits
amplified the seismic waves, or they liquefied, destroying the foundation of some
buildings. Double resonance coupling between the earthquake waves, the subsoils, and
the buildings caused intensity IX shaking in some areas, lasting up to three minutes.
Earthquakes in 1957 and in 1979 also damaged Mexico City. However, neither of these
earthquakes was quite as devastating as the 1985 earthquake. In the area of greatest
damage in downtown Mexico City, some types of structures failed more frequently than
others. In the highest damage category were buildings with six or more floors. Resonance
frequencies of these buildings were similar to the resonance frequencies of the subsoil.
Because of the unusual flexibility of Mexico City structures, upper floors swayed
as much as one meter and frequently collapsed. Differential movements of adjacent
buildings also resulted in damage. A flexible building often failed if it was held
by adjacent, more rigid lower buildings. Damage or failure often occurred where two
swaying buildings came in contact with each other. Corner buildings were also vulnerable
to damage. Lessons learned from the patterns of earthquake damage need to be applied
to prevent another disaster when an earthquake releases stress that is building in
another area-along the Mexico coast between Acapulco and Zihuatanejo.